Chapter 7: Graphical Examples with Perl/Tk- P4
Check if Servers Are up: webping
For the last example, we'll build a GUI interface that will allow us to check
and see if several web sites are running, at pre-specified intervals. Since this
action is very similar to the UNIX ping command, we call it webping. This
application would be useful to a web administrator who had to keep track of
many different web sites, and wanted to know when one was down or not
responding. We'll be utilizing the LWP::Simple module to actually ping
each site.
The code to check a site's status is as follows, where $site is a string
containing a standard URL (like http://www.ora.com):
$content = head($site);
if ($content) {
## Site is UP.
} else {
## Site is DOWN.
}

While that's pretty simple, we have to have some way to set $site to a URL.
It's not very efficient to have to type a new site on the command line each
time we want to verify the status of a site. In order to make our GUI useful,
we want to add some basic features to it.
A place to manually enter URLs would be nice, and a display of the sites we
have checked and their status would be useful. Having the program
automatically perform an update on each of the sites in the list every 30
minutes or so would be extremely useful. In that same vein, specifying the
interval would also be easier than editing the source code any time we
decide to change how often the ping happens. After we build a list of sites, it
would be nice for the program to remember them, and bring them up
automatically the next time we start the program.
Here's the final code, with most of the mentioned features represented:
#!/usr/bin/perl -w
###################################################
####################
## Webping: A program that will detect and report
whether a web site is up.
## usage: webping [ -a ] [ -i ] [ -f
] [-- [ -geometry...]]
## -a : starts prog in "autoping" mode from
beginning.

## -i : Sets the autoping interval to
## -f : Uses instead of .webping_sites
as site list
## -- is necessary to separate webping's options
from the Window
## Manager options. Allows us to utilize
GetOptions instead of
## parsing them manually (ick).
## The standard wm specs are allowed after the --
, -geometry and
## -iconic being the most useful of them.
###################################################
####################
use Tk;
use LWP::Simple;
use Getopt::Long;
The first section of the code says to use Tk, LWP::Simple, and
Getopt::Long. We chose to utilize Getopt::Long so that we wouldn't have to
parse any command-line options ourselves. As you can see from our usage

push (@intervals, $ping_interval);
}
These segments set up stuff the program should know about. There are
default values for everything they might set on the command line. We've
declared two sorting routines to be used later on. We get the options
specified by the user (if any) to put the program in automode, add or set the
interval, and determine which file to read our list of web sites from, if not
the default file.
Next comes the meat of the GUI: setting up the window, widgets, and
callbacks. webping does more complicated things than xword, so it will take
quite a bit more effort to set it all up. No matter what it does, though, it all
looks pretty much the same: creating buttons, assigning functions for them
to call, and placing the widgets in a sensible order via pack. We won't go
into too much detail about how this all happens, but here is the code:
my $mw = MainWindow->new;
$mw->title("Web Ping");
$mw->CmdLine; ## parse -geometry and etc cmd line
options.
$frame1 = $mw->Frame;
$frame1->pack(side => "bottom", -anchor => "n",

$update_bttn->configure(-state =>
'disabled');
$del_bttn->configure(-state => 'disabled');
}
} );
if (open(FH, "$site_file")) {
while () {
chomp;
$url = $_;
&add_site;
}
close FH;
}
$url = "";
Here is where we take advantage of a "remembering" file. When the
program exits, we will save the current list of sites to this file. This way,
when the program is started the next time, it looks exactly as it did the last

time we ran it--except that the program will have updated the list of sites
with the current status.
$entry->focus;
&do_automode if ($auto_mode);
MainLoop;
Off it goes! Now all that's left in our source code are the functions that we've
bound to the buttons and various actions in the GUI. Remember, this is
where the real work comes in; without these functions the GUI would just be
a bunch of flashy buttons and lists.
sub exit_program {
my @updated = $list->get(0, 'end');
if (open FH, ">$site_file") {
map { print FH "$_\n"; } @updated;
close FH;
}
exit;
}

This is how we always save the current state of the site list. The only way to
avoid running this function when exiting the application is to use the
Window Manager's close/exit/destroy commands:
sub ping_site {
## get list of indexes in listbox of those
selected.
my $site = "";
my ($content, @down);
my @selected = $list->curselection;
$mw->configure(-cursor => 'watch');
$mw->idletasks;
foreach $index (@selected) {
my $site = $list->get($index);
$site =~ s/\s.+$//; ## Strip off last
history record (if any)
$content = head($site);

}
The function ping_site( ) is called when a new site is added to update its
status. It is also called when in automode. It checks the sites selected in the
listbox. ping_site( ) is where you could put in other things to happen when a
site is down. For instance, mail the web administrator, page the
administrator with a text message, or whatever you'd like!
sub add_site {
return if ($url eq ""); ## Do
nothing, empty string
## Validate $url contains correct information
(ie a server name)
$url = "http://$url" if ($url !~ /(\w+):\/\//);
## Insert new site name into list, and make
sure we can see it.
$list->insert('end', $url);
$list->see('end');

## Select the item so that ping_site can do all
the work
$list->selection('clear', 0, 'end');
$list->selection('set', $list->size - 1);
$url = ""; ## Clear out string for next site
&ping_site;
}
We've set the default behavior of adding a site to automatically ping that
site. You could comment out that line if you didn't want to wait for the ping
to happen and you're adding a large number of sites. Remember, this would
also affect what happened when the programs started up, since this function
is called both at the beginning and during the manual adding of sites.
sub delete_site {
my @selected = $list->curselection;
## Have to delete items out of list backwards
so that indexes

## we just retrieved remain valid until we're
done.
map { $list->delete($_) } sort antinumerically
@selected;
$update_bttn->configure(-state => 'disabled');
$del_bttn->configure(-state => 'disabled');
}
The function delete_site( ) will delete any selected items in the listbox. This
allows us to remove ou-of-date sites from our list without having to edit the
.webping_sites file manually.
sub do_automode {
## State if the $automode_bttn will tell us
which way we are in.
my $state = $automode_bttn->cget(-text);
if ($state =~ /^Start/) {
$automode_bttn->configure(-text => "End
Automode");